<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        #d1 {
            color: red;
        }

        div {
            /*important优先级高*/
            color: blue !important;
        }
    </style>
</head>

<body>
<div id="d1">
    <p>这是个p</p>
    <span>这是div里面的span</span>
    <a href="">超链接</a>
</div>
<span>这是div外面的span</span>
</body>
</html>